This invention relates to a water purifying apparatus for purifying raw water such as tap water and groundwater to provide purified water for domestic or business use.
Generally, a conventional water purifying apparatus comprises a cistern and a chlorine generator placed in the cistern. The chlorine generator comprises a pair of flat electrodes serving as an anode and a cathode. In the chlorine generator, these electrodes are applied with a d.c. voltage at a predetermined interval monitored or measured by a timer to electrolyze raw water within the cistern. As a result, an appropriate amount of hypochlorous acid is produced as effective chlorine effective in sterilization.
In the above-mentioned electrolyzing operation, various chemical reactions take place. These chemical reactions are represented by:
At the cathode: EQU 2H.sup.+ +2e.sup.-.fwdarw.H.sub.2
At the anode: EQU 2Cl.sup.-.fwdarw.Cl.sub.2 +2e.sup.- 4OH.sup.-.fwdarw.O.sub.2 +2H.sub.2 O+4e.sup.-
In water: EQU Cl.sub.2 +H.sub.2 O.revreaction.HOCl+HCl
Herein, HOCl represents the hypochlorous acid.
In the conventional water purifying apparatus, however, the raw water is at first reserved in the cistern and then electrolyzed so that the raw water is processed into purified water to add hypochlorous acid therein. Thereafter, the purified water is delivered from the cistern through a pipe to a terminal unit such as a tap or a dispensing valve of a beverage dispenser. If the pipe is long, a pump is essential to feed the purified drinking water. Depending upon the length of the pipe, the pump must have an extremely large scale. Thus, the water purifying apparatus is uneconomical. Furthermore, in order to provide the purified water of a sufficient amount to meet the demand, the cistern is inevitably increased in size. As a result, it is difficult to achieve a compact water purifying apparatus.
In order to solve the above-mentioned problem, the present inventors have proposed a water purifying apparatus in U.S. patent application Ser. No. 09/168,170 (Thailand patent application No. 046502) corresponding to Japanese Patent Application No. 9-277333 (277333/1997). The water purifying apparatus is for purifying raw water containing chlorine ions to provide purified water containing effective chlorine. The water purifying apparatus comprises a water tank for reserving the raw water, a cylindrical inner electrode placed in the water tank, a cylindrical outer electrode concentrically arranged around the inner electrode to define a water channel between the inner and the outer electrodes for passing the raw water, and voltage applying means connected to the inner and the outer electrodes for applying a voltage between the inner and the outer electrodes. The voltage causes electrolysis of the raw water in the water channel to process the raw water into the purified water. A combination of the inner and the outer electrodes serves as the chlorine generator.
In the water purifying apparatus, the chlorine generator has dual functions as a cistern-type chlorine generator for processing still water reserved in the water tank during a suspension period in which water supply is suspended and as a channel-type chlorine generator for processing running water flowing through the water tank during a supply period in which the water supply is carried out. Therefore, it is possible to stably provide the purified water and to reduce the size of the water purifying apparatus as compared with the water purifying apparatus of a cistern type.
During the suspension period, however, hypochlorous acid produced between the inner and the outer electrodes tends to stay within the space defined by the inner and the outer electrodes. This results in nonuniform distribution of concentration of hypochlorous acid in the raw water reserved in the water tank. Thus, it is difficult to provide the purified water having a uniform and desired chlorine concentration during the supply period. Furthermore, oxygen gas and hydrogen gas generated by the electrolysis as resultant gases are hardly released from an outer surface of the inner electrode and an inner surface of the outer electrode. As a consequence, the resultant gases tend to stay within the space defined by the inner and the outer electrodes. This bring about decrease in efficiency of electrolysis in the chlorine generator.
Besides, when tap water as the raw water is electrolyzed by the use of the chlorine generator, the amount of effective chlorine (hypochlorous acid) generated by the electrolysis is different depending upon the chlorine ion concentration or the water temperature of the tap water.
At first, consideration will be made about the relationship between the chlorine ion concentration and the amount of effective chlorine. When the chlorine ion concentration is high and low, the amount of effective chlorine is great and small, respectively. It is noted here that the tap water in a big city generally relies upon a surface water of a river as a water resource and therefore exhibits a wide range of daily or hourly variation of the chlorine ion concentration. Accordingly, the amount of effective chlorine generated by the electrolysis also exhibits a wide range of such daily or hourly variation. Under the circumstances, it is difficult to continuously keep the amount of effective chlorine at a desired value in the purified water obtained by electrolyzing the tap water of the above-mentioned origin. Furthermore, the range of daily or hourly variation in chlorine ion concentration of the tap water is greatly different depending upon a regional difference of a water quality. This also makes it difficult to continuously keep the amount of effective chlorine in the purified water at a desired value.
Next, the relationship between the water temperature and the amount of effective chlorine will be considered. When the water temperature is high and low, the amount of effective chlorine is great and small, respectively. If the water temperature of the tap water exhibits a wide range of daily or hourly variation, the amount of effective chlorine generated by the electrolysis also exhibits a wide range of daily or hourly variation. Under the circumstances, it is difficult to continuously keep the amount of effective chlorine at a desired value in the purified water obtained by electrolyzing the tap water which is wide in temperature variation. Furthermore, the range of daily or hourly variation in water temperature of the tap water is greatly different depending upon a regional or a seasonal difference. This also makes it difficult to continuously keep the amount of effective chlorine in the purified water at a desired value.
In order to overcome the above-mentioned difficulties, it is proposed to energize the electrodes with an output level of a power supply controllably changed in correspondence to the water quality or the water temperature. In this manner, an appropriate chlorine concentration is obtained. However, changing the output level in proportion to the water quality or the water temperature inevitably results in an increased scale of the power supply and in high cost.